Below-knee or above-knee, leg amputated persons may nowadays regain some measure of locomotion autonomy by installing a prosthetic limb to their stump. The foot part of the prosthetic limb will usually be an add-on element to be fixedly yet releasably secured to the prosthetic leg part proper.
First generation prosthetic feet implements included feet articulated at the ankle, and others forming a rigid segment with the shank segment. These implements addressed the needs of amputees with respect to joint mobility or greater level of transportation autonomy.
Second generation prosthetic feet appeared as deformable foot frames or keels were provided, wherein the energy of deformation stored during heel strike (the foot heel initially engaging the ground) could be, at least in part, restored during the foot push-off period (just before foot rise), solely in the sagittal plane, in order to drive the artificial limb forward in the sagittal plane. Some of these second prosthetic feet are disclosed in the following United States patents:
(A) U.S. Pat. No. 4,547,913 issued on Oct. 22, 1985 to Flex Foot inc. (inventor V. Phillips).
(b) U.S. Pat. No. 4,645,509 issued on Feb. 24, 1987 to the Model & Instrument Development corp. (inventors: D. Poggi, E. Burgess, D. Moeller and D. Hittenberger)--the so-called "Seattle foot"--;
These prosthetic foot keels extend within a plane disclosing substantial symmetry with the lengthwise axis and sagittal plane of the prosthetic leg. The mechanical behaviour describing the deformation and energy storing and release of these second generation prosthetic feet have been well studied by those skilled in the art. Although these artificial feet were immediately heralded by users as a major improvement in comfort level, it was obvious from observations and results that walking cadence was still much lower and stride length, still far shorter than the natural cadence and stride length of unamputated persons. Such second generation implements could therefore be improved, to bring the amputee's gait more closely in line with unamputated person's gait.
Eventually, it became clear that inadequate medio-lateral control of the amputee's foot was at least in part to blame, i.e. the cyclic requirement of lateral back and forth swinging motions of the amputee's body center of weight from one leg to the other, during gait. Moreover, there was still too static or passive contribution of the prosthetic foot energy restoring, forward thrust capability at push-off (the "dead-leg" syndrome). Indeed, these artificial limbs did disclose at least fair to good forward and vertical push-off thrust capability, due to their spring-back capability and symmetrical shape about the sagittal plane of the leg, but the medio-lateral contribution was relatively limited. This is because for the latter to be achieved, it was required to pivot by internal rotation the whole foot about the substantially vertical, leg lengthwise axis, rather than through laterally inward roll motion of the prosthetic foot about its horizontal fore and aft axis, hingedly about the ankle, as is typically the case for healthy, full length, natural legs.
Moreover, it was recognized in the field that the much touted "Seattle foot", although efficient during athletic movements performed by the amputee, was much less advantageous for day to day normal walking. Indeed, it is recognized in the medical field that a majority of leg amputees, perhaps as much as 95% of them, can link their handicap to a prior, triggering vascular trauma usually associated with a heart attack. Such vascular trauma occurs overwhelmingly in the case of elderly patients, who normally would not engage into physically demanding sporting activities. The efficiency of the "Seattle foot" is low because, at slow cadence gait, the value of strained energy stored by keel flexion at heel strike, and following spring back energy restoration at toe-off, is typically only about 1.5 times the body weight of the amputee, whereas, at fast-cadence gait, said value is doubled to about 3 times the body weight of the amputee. Thus, spring back of the "Seattle foot" keel would be low if not negligible in the case of an elderly amputee, presenting very short stride lengths typical of a hesitant person who experiences trouble maintaining his stability over ground. Therefore, the "Seattle foot" for such amputees would not constitute an improvement over said first generation prosthetic limbs.